Binary Tellurium(IV) Azides: Te(N3)4 and [Te(N3)5]

Authors


  • Financial support from the University of Munich and the Fonds der Chemischen Industrie is gratefully acknowledged. We thank Prof. P. Klüfers for generous allocation of X-ray diffractometer time, and Prof. K. O. Christe and his colleagues for exchange of information about their independent syntheses of Te(N3)4 and [Te(N3)6]2−.

Abstract

original image

Very sensitive materials: Tellurium tetraazide, Te(N3)4, was prepared directly from TeF4 and Me3SiN3 as an extremely sensitive solid; azidation of a pentafluorotellurate(IV) anion gave the pentaazidotellurate(IV) anion. The crystal structure of the pyridinium salt [pyH][Te(N3)5] consists of [Te(N3)5] units, considerably distorted from ideal square-pyramidal symmetry, that are linked through Te⋅⋅⋅N interactions (see picture).

Apart from some early reports on the elusive and extremely labile tellurium nitride compounds TeN, Te3N4, and Te4N4, and the recently discovered [Te6N8(TeCl4)4],1 the only structurally characterized binary Te–N species is the salt [Te(N3)3][SbF6].2 The triazidotelluronium cation was obtained as an unexpected product during an attempted preparation of [Te2N]+. The chemistry of tellurium azides was initiated by Wiberg in the 1970s.3 Various examples of homoleptic azido transition and main-group metalates and nonmetalates are reported,4 but none of the chalcogen group. The groups of Wiberg and Passmore have indicated the possible existence of a highly explosive Te(N3)4 and warned of their potential danger. In pursuit of our recent efforts to explore the chemistry of covalent and ionic tellurium azides,5, 6 we describe here the synthesis, isolation, and properties of Te(N3)4 (1),7 [Me4N][Te(N3)5] (2 a) and [pyH][Te(N3)5] (2 b), the latter containing a rare pentacoordinated polyazido anion, the only other example being the [Fe(N3)5]2− ion.8

As is known from the syntheses of TeCl3(N3) and TeCl2(N3)2,3 and confirmed by our own studies, treatment of TeCl4 with excess Me3SiN3 does not result in the substitution of all chlorine atoms. All successful preparations for R2Te(N3)2 and RTe(N3)3 (R=alkyl, aryl, trifluoromethyl and perfluoroaryl) proceed via the corresponding tellurium di- or trifluorides R2TeF2 and RTeF3.5, 6a Thus, for the synthesis of Te(N3)4 (1) and [Te(N3)5] (2), the fluorinated species TeF4 and [Me4N][TeF5] were preferred as starting materials. Tellurium tetrafluoride rapidly reacts with Me3SiN3 in CFCl3 (0 °C) suspension to form a yellowish precipitate of 1 that is soluble in DMSO [Eq. (1)((1))]. On one occasion, the solid exploded violently when obtained from a suspension in CH2Cl2. Solutions of 1 in [D6]DMSO exhibit a very broad 125Te NMR resonance at δ=1380 ppm, deshielded compared to that of TeF4 (δ=1195 ppm). The resonance for 1 is identical to that detected in a mixture of the dismutation products of C6F5Te(N3)3, Te(N3)4, and (C6F5)2Te(N3)2.6a Due to our experience with the unpredictable explosiveness of neat 1, vibrational spectra, mass spectra, and elemental analysis were omitted.9

equation image((1))

In a similar fashion, [Me4N][TeF5] reacts with Me3SiN3 in CH2Cl2 to form a yellow solution of [Me4N][Te(N3)5] (2 a) [Eq. (2)((2))], from which yellow crystals can be grown at below −20 °C. In some cases, an extremely sensitive yellow oil also separates, which in one case exploded when the cold mixture was stirred. For 2 a, a sharp 125Te NMR resonance was observed at δ=1258 ppm (CD2Cl2), again downfield from that of [TeF5]. The 125Te NMR resonance of [TeF5] was observed at 25 °C in CH2Cl2 solution in the presence of [Ph4P]+ ions as a doublet of quintets at δ=1161 ppm (1J(125Te-Fapical)=2918 Hz; 1J(125Te-Fbasal)=1386 Hz). The Raman spectrum of 2 a shows bands at 2105/2055 [νas(N3)] and 409/347 [ν(TeN)] cm−1, which are detected in the typical regions for tellurium azides.5, 6 A reaction between [Me4N]2[TeCl6] and Me3SiN3 did not occur, and the [TeF6]2− ion remains still unknown.10

equation image((2))

In an attempt to stabilize the tetraazide 1, and to gain products more easily characterizable, an effort was made to react a pyridine⋅TeF4 adduct with azide. Such TeF4 adducts are reported, but except for elemental analysis, no analytical information is available.11 In order to confirm that compounds of the proposed form [L⋅TeF3][TeF5] (L=Me3N, pyridine etc.) were prepared,12 further analytical information was desirable. In the course of the preparation of such a proposed pyridine⋅TeF4 adduct, the results of the crystal-structure determination unexpectedly revealed a complex mixture of pyridinium pentafluorotellurate(IV) and dimeric units of TeF4 solvated by pyridine.13

Since crystals of 2 a grown from the reaction solution tend to deliquesce very rapidly, crystallization was attempted from solutions of the mixture of pyridine with TeF4 and Me3SiN3. Although the crystals obtained after several weeks were shown to be the pyridinium salt [pyH][Te(N3)5] (2 b), probably formed by reaction of TeF4 with fluoride in the employed glass vessel to give [TeF5] ions, the resulting pentaazidotellurate(IV) anion was unaffected. The pyridinium salt 2 b crystallizes in the triclinic space group Pequation image, for the anion a distorted Ψ-octahedral TeEN5 coordination is found (Figure 1). The [Te(N3)5] ion represents the first structurally characterized anionic tellurium azide.7 Similar to neutral organotellurium(IV) azides,5, 6a secondary Te⋅⋅⋅N interactions below the sum of the van der Waals radii (3.61 Å)14 create network structures, which result in the octacoordinated tellurium atoms in 2 b (Figure 2). The Te[BOND]N bond lengths vary from 2.075(2) Å (Te-N7) for the apical azide group, to between 2.175(2) and 2.256(2) Å for the basal azide groups. The apical N3 unit has the shortest Nα[BOND]Nβ/Nβ[BOND]Nγ bond lengths and the smallest N-N-N angle. The N-Te-N bond angles range between 74.53(7) and 165.99(8)°, and the N-N-N bond angles are 175.8(2)–177.9(3)°. The rather irregular square-pyramidal structure likely results from electrostatic repulsions of the differently polarized Nα and Nβ atoms of the azide groups, both between each other, and with the lone pair of the TeIV center. Two sets of virtually identical Te[BOND]N distances for two geminal basal azide groups are present with four different orientations towards the apical position (Figure 1).

Figure 1.

Molecular structure of the anion in 2 b with thermal ellipsoids at 50 % probability. Selected bond lengths [Å] and angles [°]: Te-N1 2.185(2), Te-N4 2.256(2), Te-N7 2.075(2), Te-N10 2.175(2), Te-N13 2.242(2); N1-N2-N3 176.9(2), N7-N8-N9 175.8(2), N1-Te-N7 85.34(8), N7-Te-N13 74.53(7).

Figure 2.

Secondary Te⋅⋅⋅N interactions between [Te(N3)5] ions in 2 b. Selected separations [Å]: Te⋅⋅⋅N3(i) 3.324(2), Te⋅⋅⋅N4(i) 3.227(2), Te(ii)⋅⋅⋅N10 3.127(2). Symmetry operations: i=1−x, 1−y, −z; ii=−x, 1−y, −z.

The electronic structure of the [Te(N3)5] ion was calculated using different ab initio and density functional methods and basis sets (Table 1). All geometries have been fully optimized at the level chosen and led to distorted C1-symmetric minimum structures in good agreement with the experimental crystal-structure determination. Best results regarding bond lengths and vibrational frequencies were obtained by using the B3LYP/SDD combination. However, this finding may be due to accidental compensation of pseudopotential and basis deficiencies, which may suggest a higher degree of accuracy for this cheaper method.

Table 1. Experimental and calculated parameters of [Te(N3)5].
Method experimentE [Hartree]zpe [kcal mol−1]d(Te-Napical) [Å]d(Te-Nbasal) [Å]equation imageas(N3) [cm−1][a]
  1. [a] Vibrational frequencies are unscaled.

 2.0752.175–2.2562112–2037
RHF/SDD−823.79141.02.0582.108–2.2182311–2203
MPW1PW91/SDD−828.64237.92.1242.159–2.2182133–2097
B3LYP/SDD−828.87736.72.1402.179–2.2452067–2030
MP2(FC)/SDD−825.81536.52.1132.156–2.2382275–2144
RHF/SDB-cc-pVTZ20b−824.53844.72.0202.098–2.2092488–2369
B3LYP/SDB-cc-pVTZ20b−829.44039.92.0772.159–2.2292226–2171

After the structural characterization of binary Te azide species of the type [Te(N3)3]+2 and [Te(N3)5] (2 a and b) and the first direct synthesis and NMR spectroscopic characterization of the neutral tellurium azide molecule, Te(N3)4 (1), the race is now on for the isolation of the first selenium azide species. Studies in this regard are currently in progress.15

Experimental Section

All manipulations of air- and moisture-sensitive materials were performed under an inert atmosphere of dry argon using flame-dried glass vessels and Schlenk techniques. Tellurium tetrafluoride and [Me4N][TeF5] were prepared according to the literature,16 trimethylsilyl azide (Aldrich) was used as received. Solvents were dried by standard methods, distilled, and stored over molecular sieves. Raman spectra were recorded on a Perkin Elmer 2000 NIR FT spectrometer fitted with a Nd-YAG laser (1064 nm). NMR spectra were recorded on a JEOL Eclipse 400 instrument, and chemical shifts were referenced to CH3NO2 (14N) and Me2Te (125Te).

CAUTION: Binary tellurium azides are extremely hazardous shock-, friction-, and moisture-sensitive materials, which can explode unexpectedly. Appropriate safety precautions, such as Kevlar gloves (Sahlberg, Munich), face shield, leather jacket (Quadratfuss, Berlin), and teflon spatulas (Merck, Darmstadt; customized by LMU) must be employed; the quantities of substances handled in pure form by experienced personnel should not exceed 100 mg.

1: A suspension of TeF4 (0.3 mmol, 60 mg) in CFCl3 (10 mL) was treated with Me3SiN3 (1.2 mmol, 150 mg) at 0 °C. After stirring for 2 h the supernatant was decanted. The residue was dissolved in [D6]DMSO for NMR experiments. 14N NMR (28.9 MHz, [D6]DMSO, 25 °C; Δν1/2 (Hz)): δ=−141 (80, Nβ), −234 (1000, Nγ), ≈−270 ppm (extremely broad, Nα); 125Te NMR (126.1 MHz, [D6]DMSO, 25 °C): δ=1380 ppm.

2 a: A solution of [Me4N][TeF5] (0.15 mmol, 45 mg) in CH2Cl2 (5 mL) was reacted with Me3SiN3 (0.75 mmol, 85 mg) at 0 °C. After a few minutes, a yellow solution formed. After removal of all volatile materials in vacuo, a yellow solid remained. Raman (100 mW, 25 °C): equation image=3037(20), 2979(20), 2921(20), 2105(40)/2055(15) [νas(N3)], 1472(10), 1459(10), 1447(15), 1312(15) [νs(N3)], 1263(10), 948(15), 753(20), 668(10), 639(20), 409(100)/347(85) [ν(TeN)], 255(30), 203(40), 174(50) cm−1; 14N NMR (28.9 MHz, CD2Cl2, 25 °C, Δν1/2 (Hz)): δ=−139 (20, Nβ), −236 (270, Nγ), ≈−250 (extremely broad, Nα), −337.8 ppm (3, Me4N); 125Te NMR (126.1 MHz, CD2Cl2, 25 °C): δ=1258 ppm.

2 b: A suspension of TeF4 (1 mmol, 205 mg) in dry pyridine (5 mL) was stirred for 12 h. All volatile materials were removed in vacuo and a part of the obtained residue (100 mg) was treated with Me3SiN3 (1.1 mmol, 125 mg) at 0 °C. After stirring for 1 h, the resulting yellow mixture was decanted. Upon cooling at −25 °C for 30 min, yellow crystals of 2 b can be carefully separated from the supernatant. Raman (100 mW, 25 °C): equation image=3109(5), 3052(5), 2112(30)/2079(10)/2064(5)/2037(15) [νas(N3)], 1640(2), 1618(2), 1490(2), 1323(10) [νs(N3)], 1298(5), 1272(5), 1245(5), 1200(5), 1029(10), 1011(20), 667(5), 651(5), 639(10), 406(100)/342(50) [ν(TeN)], 300(20), 253(15), 193(35), 170(30) cm−1. 14N NMR (28.9 MHz, CDCl3, 25 °C, Δν1/2 (Hz)): δ=−115 (400, py), −141 (25, Nβ), −234 (360, Nγ), −245 ppm (extremely broad, Nα); 125Te NMR (126.1 MHz, CDCl3, 25 °C): δ=1334 ppm.

Crystal data for 2 b: C5H6N16Te (Mr=417.81), yellow block, 0.14×0.22×0.30 mm, triclinic, space group Pequation image, a=7.6459(1), b=10.3875(1), c=10.5173(2) Å, α=117.6238(8), β=91.5610(9), γ=107.847(1)°, V=690.13(2) Å3, Z=2, ρcalcd=2.011 g cm−3, μ=2.184 mm−1, F(000)=400, Nonius Kappa CCD, Mo (λ=0.71073 Å), T=200 K, θ range=3.35 to 27.52°, −9≤h≤9, −13≤k≤13, −13≤l≤13, reflections collected: 10 103, independent reflections: 3134 (Rint=0.0376), observed reflections: 2940 [I>2σ(I)], structure solution: SIR97,17 direct methods, data to parameters ratio: 14.0:1 [13.2:1 I>2σ(I)], final R indices: R1=0.0228, wR2=0.0545 for [I>2σ(I)]; R1=0.0257, wR2=0.0554 for all data, GOF on F2=1.160.

CCDC-215495 (2 b) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or deposit@ccdc.cam.ac.uk).

Ab initio calculations: Gaussian03 program package,18 HF-SCF, DFT, and MP2 methods, basis sets (N: Dunning/Huzinaga valence double-zeta, cc-pVTZ basis sets;19 Te: double-zeta or triple-zeta basis sets for the valence electrons and energy-consistent large-core ECPs for 46 core electrons20) as implemented in Gaussian.

In memory of Marianne Baudler

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